Download Methods of Strengthening Ceramics

Sanjay Madhavan, et al /J. Pharm. Sci. & Res. Vol. 7(10), 2015, 873-877
Methods of Strengthening Ceramics
1
Sanjay Madhavan, 2Dr. Anand
1 nd
2 year BDS , Saveetha dental college.
Department of prosthodontics,Saveetha dental college
2
Abstract:
Aim
To know about the various methods of strengthening ceramics.
Objective
To provide information on ceramic dental materials and various techniques to strengthen ceramics.
Background
Ceramics are defined as solids with a mixture of metallic or semi-metallic and non-metallic elements that are quite hard, nonconducting and corrosion-resistant. Today's ceramic restoration application, despite its popularity gained through esthetical
advantages and superior hygienically features, has a fragile structure due to its low tensile strength quality. In the long run, it
can lose its endurance against shear and draw strengths that occur while chewing. Technical flaws like micro cracks and
porosity which appears during production causes mechanical failure in porcelain restoration. This article reviews the concepts
and mechanisms of strengthening ceramics.
Reason
To review about the various methods of strengthening ceramics for improving the results.
Keywords: ceramics, tempering, stress, transformation
INTRODUCTION
In dentistry, ceramics represents one of the four major
classes of materials used for the reconstruction of decayed,
damaged or missing teeth. The other three classes being
metals, polymers and composites. The word Ceramic is
derived from the Greek word “keramos”, which literally
means ‘burnt stuff’, but which has come to mean more
specifically a material produced by burning or firing [1]. A
ceramic is an earthly material usually of silicate nature and
may be defined as a combination of one or more metals
with a non-metallic element usually oxygen. The American
Ceramic Society has defined ceramics as inorganic, nonmetallic materials, which are typically crystalline in nature,
and are compounds formed between metallic and nonmetallic elements such as aluminium & oxygen, calcium &
oxygen, silicon & nitrogen [2]. The term porcelain is
referred to a specific compositional range of ceramic
materials made by mixing kaolin, quartz and feldspar in
proper proportioning and fired at high temperature [3,4].
Porcelain is essentially a white, translucent ceramic that is
fired to a glazed state. [5]
In dentistry, ceramics are widely used for making artificial
denture teeth, crowns, bridges, ceramic posts, abutments,
and implants and veneers over metal substructures [6].
Ceramic material used in dentistry is a glassy matrix that is
produced via sintering and it contains leucite crystals; it can
be identified as a sort of ceramic that is not totally
transformed into glass phase. Ceramics are characterized by
their refractory nature, hardness, chemical inertness,
biocompatibility and susceptibility to brittle fracture [7,8].
Crystal structure of ceramic material contains ionic and
covalent type of strong bonds that gain stability and
determination. However, because of those bonds, ceramic
has a fragile structure. Ceramic has a high rigidity and
pressure strength and low flexion performance [9]. The
pressure strength of dental ceramics is between 350-550
MPa and the draw strength value is 20-60 MPa. That is
why ceramic structure should be supported either by metal
or substructures more durable which decrease effects of
tension strength occurring on surface or should be
strengthened structurally.[10,11]
CLASSIFICATION OF CERAMICS
Composition;
Pure alumina, pure zirconia, silica glass, leucite-based glass
ceramics, lithia-based glass ceramics.
Processing methods;
Sintering, partial sintering and glass infiltration, heat
pressing, slip casting, CAD/CAM and copy milling.
Core;
In-ceram spinell
In-ceram alumina
In-ceram zirconia
Firing temperature;
High fusing (1,300°C), medium fusing (1,101-1,300°C),
low fusing (850-1,000°C), ultra low fusing (below850°C).
Substructure material;
Feldspathic porcelain, cast metal, swaged metal, glass
ceramics, CAD/CAM, sintered ceramic core.
Microstructure;
Amorphous glass, crystalline porcelain, crystal containing
glass, partially crystallized porcelain and apatite ceramic.
Translucency;
Opaque, translucent, transparent.
Indications;
Anterior and posterior crowns, veneers, posts and cores,
FPDs, stain ceramic, glaze ceramic, metal ceramic, inlays,
onlays and implants.[12]
873
Sanjay Madhavan, et al /J. Pharm. Sci. & Res. Vol. 7(10), 2015, 873-877
COMPOSITION OF CERAMIC
Feldspar - it is the lowest fusing component, which melts
first and flows during firing, initiating these components
into a solid mass.
Silica (Quartz)- Strengthens the fired porcelain restoration,
remains unchanged at the temperature normally used in
firing porcelain and thus contribute stability to the mass
during heating by providing framework for the other
ingredients.
Kaolin- binder, increases moldability of the unfired
porcelain, imparts opacity to the finished porcelain product.
Glass modifiers, e.g. K, Na, or Ca oxides or basic oxidesthey interrupt the integrity of silica network and acts as
flux.
Colour pigments or frits, e.g. Fe/Ni oxide, Cu oxide, MgO,
TiO2, and Co oxide- to provide appropriate shade to the
restoration.
Zr/Ce/Sn oxides and Uranium oxide.[13]
PROPERTIES
Dental ceramics exhibit excellent biocompatibility with the
oral soft tissues and are also chemically inert in oral cavity.
They possess excellent aesthetics. Ceramics are good
thermal insulators and their coefficient of thermal
expansion is almost close to the natural tooth [14,15].
Dental ceramics possesses very good resistance to the
compressive stresses, however, they are very poor under
tensile and shear stresses. This imparts brittle nature to the
ceramics [16,17,18] and tend to fracture under tensile
stresses. Structural defects lead to the failure in dental
ceramic prostheses. Defects may arise in the form of microcracks of sub-millimeter scale; during fabrication of
ceramic prostheses and also from application of
masticatory forces in the oral cavity [19]. Fatigue strength
plays an important role in the durability and longevity of
dental ceramic restorations.
METHODS OF STRENGTHENING CERAMICS
The major drawbacks of ceramics are brittleness, low
fracture toughness and low tensile strength. Methods used
to overcome the deficiencies of ceramics fall into two
categories including methods of strengthening brittle
materials and methods of designing components to
minimize stress concentration and tensile stress [20].
The approaches in strengthening ceramics are as follows;
1. Shot peening
2. Strengthen with a metal substructure
3. Dispersion strengthening of glasses
4. Enamelling of high strength crystalline ceramics
5. Controlled crystallization of glasses
6. Production of prestressed surface layers in dental
porcelain via ion exchange, thermal tempering.
7. Optimum Restoration Design
8. Crack tip blunting
9. Transformation Saturation
10. Minimizing the number of firing cycles
SHOT PEENING
Shot peening is a type of surface treatment used to
strengthen ceramics. It is a cold working process that
shoots balls (shot) of steel, ceramics or glass beads at the
workpiece to mechanically prestress the material surface
beyond its yielding point[21]. The localized plastic
deformation induces residual stresses into the surface
region of the material. The surface residual stresses are
compressive. The induced compressive residual stresses
inhibit crack growth under both static and cyclic loading,
increasing the material hardness, fatigue life and resistance
to stress corrosion cracking.
STRENGTHEN WITH A METAL SUBSTRUCTURE
The restoration system which involves strengthening the
ceramic material with metal was firstly invented by
Weinstein in 1962. Metal ceramic systems were developed
to reinforce the ceramics[22]. They are:
1.
2.
Noble metal alloy systems (high gold, low gold, gold free).
Base metal alloy systems (NiCr,Ti)
In order to strengthen dental ceramic and to improve its
strength against tension, shear and pressure forces,
generally a metal substructure is used. In a ceramic that
sub-structurally improved by metal, micro fractures spread
if only this strong substructure gets deformed. The
perimeter of thermal dilatation of substructure material
should be higher than porcelain's. During cooling process,
metal substructure shrinks more and more and it supports
porcelain with pressure generated [23]. Earlier methods
employed to enhance bonding with precious metals were
coating with tin oxide. Platinum copings were electroplated
with a layer of tin oxide to which aluminous porcelain was
attached.
Twin foil technique involves laying down of two platinum
foils in close opposition to each other foil provides a matrix
for the bonding of the porcelain which is removed after
baking. The outer foil forms an inner skin to the crown. It is
tin plated and oxidized to achieve a strong chemical bond
with the aluminous core porcelain[24].
Noble metal foils are adapted, swaged and brazed on to
dies and then bonded to feldspathic porcelain [25]. The
advantages here include reduction of metal and labour cost,
a porcelain butt fit, avoidance of metal collar, less stresses
at the porcelain metal interface, reduction of internal micro
cracks and subsurface porosity, so lesser sites of crack
propagation.
DISPERSION STRENGTHENING OF GLASSES
Dental ceramics that contains glass phase can be
strengthened by dispersion strengthening i.e. dispersing
ceramic crystals of high strength and elasticity such as
leucite, lithium disilicate, alumina, magnesia-alumina,
spinel, zirconia in the glass matrix. When crystal materials
are added in the glassy phase, a strong glass-crystal
composition is obtained, thus durability and fracture
resistance increases. Crystal particles prevent micro
fractures to push on forward and it provides a strong
structure[26].
Limiting factors while choosing reinforcing crystals are
fusion temperature, coefficient of thermal expansion,
bonding properties with dental porcelain, mechanical
strength and resistance to thermal shock during rapid firing
cycles.
874
Sanjay Madhavan, et al /J. Pharm. Sci. & Res. Vol. 7(10), 2015, 873-877
Quartz (10-15%) undergoes changes during heating and has
a high coefficient of thermal expansion and the
strengthening effect of quartz is poor.
Alumina reinforcement: When alumina crystals are
dispersed in a glass matrix and heated and cooled, different
stress patterns are observed due to the differences in
thermal expansion between glass and alumina[27].
ENAMELLING
CERAMICS
OF
HIGH
STRENGTH
CRYSTALLINE
During firing some form of crystallization takes place in
ceramics (sintered or high alumina), resulting in an
interlocking crystalline system which is better able to
withstand high stresses than feldspathic porcelain. High
alumina cores with aluminous porcelain veneers have been
used in combination. These laminates are much stronger
than regular porcelain, similar to metal ceramic systems.
The bonding at the interface is chemical in nature and an
ionic bond ensures no porosity as the wetting of the
porcelain enamel on high alumina is good[28].
CONTROLLED CRYSTALLIZATION OF GLASSES
Under normal conditions, when a glass is heated up to a
determined degree and then get cooled down, it does not
crystallize. In this method, ceramic structure is heated up to
first softening temperature, made to crystallize by adding a
nucleating agent like titanium dioxide, lithia, zinc oxide,
silica or metal phosphates[29]. Though the glass is amber
in colour and glassy it becomes translucent and tooth like
after crystallization or ceramming for 1 hour at 600°C.
Here high thermal shock resistance and improved strength
property has been observed.
ION EXCHANGE (CHEMICAL TEMPERING)
In general, ceramic restorations fail because of larger and
deeper micro fractures caused by tensile strength. Ion
exchange method is to generate at low temperature a
compressive layer on ceramic's surface in order to micro
fractures becomes larger. This compressive layer on surface
is created by exchange of some ions with bigger ions of
glass matrix. Dental ceramic material is plunged into
melted potassium nitrate salt tank cooler than glass
transition temperature and Na+ ions found on dental
ceramic's surface change place with K+ ions of salt tank.
By way of compressing on silicate system, Potassium ions
which are bigger than sodium ions, generate a compression
power [30,31]. This surface compression gives an increase
in strength on the surface of porcelain.
THERMAL TEMPERING
Rapid cooling or quenching of a surface of an object while
it is still hot creates residual surface compressive stresses
on the surface of the ceramics. As the core is hot and soft
and still in its molten state it tends to shrink and tries to pull
the outer surface which is rigid now. On solidification,
residual tensile stresses are created on the inner core and
residual compressive stresses on the outer surface [32]. Hot
glass phase ceramics are quenched in silicone oil or other
special liquids [33].
OPTIMUM RESTORATION DESIGN
Before designing a ceramic restoration which will cope
with every negative condition, ceramic's weakness against
low tensile strength, its fragility and sensitivity to micro
fractures should be considered. In this design, ceramic
should be protected from high tension. Avoid sharp edges
and apparent thicknesses from restorations [34,35]. Best
way to decrease the tensile strength on bridges is to design
connector zones that have intense stress with an appropriate
thickness and shape [36]
Minimizing stress concentrators and stress raisers: Stress
raisers are discontinuities in ceramic restoration that can
cause stress concentration. The design of the ceramic
should avoid these stress concentrators. Abrupt changes in
contour including any grooves, pits, notches can alter the
stress flow lines. The internal angles in tooth preparation
should not be sharp but rounded.
CRACK TIP BLUNTING
Porcelains are solid materials that have a very small work
of fracture[37,38,39].They will tolerate cracks much deeper
than 0.025 mm but when the crack propagates its tip radius
remains the same throughout the length and very little force
is required to propagate the stress. Once porcelain is under
tension, the crack propagates and a complete fracture
occurs suddenly. Crack tip blunting is a reinforcing
mechanism. The principle is somewhat perplexing in that
hollow spaces are actually used to strengthen ceramics. As
the crack progresses it is dissipated into the void space. The
stress is usually increased at the narrow tip of the crack
which is reduced at the voids and this prevents crack
propagation[40].
TRANSFORMATION SATURATION
Transformation saturation is a phenomenon, based on a
phase transformation principal caused by tension strength.
In transformation saturation method mostly leucite and
zirconium is used to strengthen the ceramic. In this
method, changes of temperature in ceramic material play an
important role. During thermal changes, volumes of leucite
and zirconium found in glassy phase increases in glassy
matrix and it creates pressure stresses inside the structure.
This pressure stresses both prevent micro fractures to push
on and does decrease tension stresses situated at
microfracture's peak [41].
Pure zirconia can exhibit a polymorphic phase
transformation.
1. Monoclinic (P21 /c)- from room temperature to heating
to 1,170°C
2. Tetragonal (p42 /nmc)- 1,170°C to 2,370°C
3. Cubic (fm3/m)- above 2,370°C till the melting point.
On cooling from 950°C there is an increase in volume
leading to a catastrophic failure. Pure zirconia is alloyed
with stabilizing oxides, such as CaO, MgO, Y2O3 or CeO2
which allows the tetragonal structure to remain stable at
room temperature, thus efficiently arresting the crack
propagation and leading to high toughness [42,43].
875
Sanjay Madhavan, et al /J. Pharm. Sci. & Res. Vol. 7(10), 2015, 873-877
MINIMIZING THE NUMBER OF FIRING CYCLES
The purpose of porcelain firing procedure is to densely
sinter the particles of powder together and produce a
relatively smooth, glassy layer on the surface. Several
chemical reactions occur over time at porcelain firing
temperature: of particular importance is increase in
concentration of crystalline leucite. Changes in the leucite
content caused by multiple firing can alter the co efficient
of thermal contraction of some porcelain products and
produce stresses during cooling, sufficient to cause crack
propagation in the porcelain [44].
OTHER METHODS TO IMPROVE STRENGTHENING ARE:
• Good condensation techniques (powder condensation),
programmed firing schedules, high pressure
compaction, vacuum fired porcelain and better
condensation in the wet stage which are all very
essential to minimize shrinkage and avoid excessive air
bubbles.
• If the surface is undisturbed, the strength of the glazed
surface specimen is found to be higher.
• Thermal stresses occurring during improper cooling can
cause cracks and weaken the porcelain. Water (saliva)
can act as a network modifier and weaken the
structure[45].
CONCLUSION
It is apparent that ceramics as a material group would
continue to play a vital role in dentistry owing to their
natural aesthetics and sovereign biocompatibility with no
known adverse reactions. However, there will always
remain a compromise between aesthetics and
biomechanical strength. In order to achieve adequate
mechanical and optical properties in the final porcelain
restoration, the amount of glassy phase and crystalline
phase should be optimised. Good translucency requires a
higher content of the glassy phase and good strength
requires a higher content of the crystalline phase. Hence,
the two material phases need to be balanced. Even though
the material is high abrasion resistant, fracture toughness
and resistance to the tensile stresses are inherent
disadvantages. To prevent these clinical problems we must
strengthen porcelain material using appropriate methods. If
dental ceramics have strength structure, dental ceramics
will become more indispensable material than amalgam
and composite for all-round restorations.
[1]
[2]
[3]
[4]
REFERENCES
Rama Krishna Alla, Dental Materials Science, Jaypee Brothers
Medical Publishers Pvt Limited, New Delhi, India, 2013, 1st Edition,
333-354.
Sukumaran VG, Bharadwaj N, Ceramics in Dental Applications,
Trends Biomater. Artif. Organs, 20(1), 7-11, Jan 2006.
Jithendra Babu P, Rama Krishna Alla, Venkata Ramaraju Alluri,
Srinivasa Raju Datla, Anusha Konakanchi, “Dental Ceramics: Part I
- An Overview of Composition, Structure and Properties.” American
Journal of Materials Engineering and Technology, 3(1): 13-18, Mar
2015.
Anusavice KJ, Phillip’s Science of Dental Materials, Elsevier, A
division of Reed Elsevier India Pvt Ltd, New Delhi, India, 2010,
11th Edition, 655-720.
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
Hämmerle C, Sailer I, Thoma A, Hälg G, Suter A, and Ramel C,
Dental Ceramics: Essential Aspects for Clinical Practice,
Quintessence, Surrey, 2008.
Denry I, Holloway JA, Ceramics for dental applications: A Review,
Materials, 3, 351-368, Jan 2010.
Garber DA, Goldstein RE, Porcelain and Composite Inlays and
Onlays: Esthetic Posterior Restorations, Quintessence, Chicago,
1994.
Touati B, Miara P, Nathanson D, Esthetic Dentistry and Ceramic
Restorations, Martin Dunitz, London, 1999.
McLean, J.W., Hubbard, J.R., Kedge, M.I. “The nature of dental
ceramics and their clinical use,” The Science and Art of Dental
Ceramics. Illinois: Quintessence Publishing Co. 1979 (1st ed.)
McCabe, J.F., Walls, A.W.G.Applied Dental Materials UK:
Blackwell Publishing Ltd. 2008 (9th ed.). 64-77.
Campbell, S.D. “A Comparative Strength Study of Metal Ceramic
and All-Ceramic Esthetic Materials: Modulus of Rupture,” J Prosthet
Dent, 62 (4), 476-479. 1989.
From Ceramics to Ceramic Steel: Genesis. Udita S Maller, Deepa
Natesan Thangaraj, Sudhakar Maller. 10.5005/jp-journals-100261030 REVIEW ARTICLE
Sakaguchi RL, Powers JM, Craig’s Restorative Dental Materials,
Elsevier, Mosby, A division of Reed Elsevier India Pvt Ltd, New
Delhi, India, 2007, 12th Edition, 443-464.
Vallittu PK Non-metallic biomaterials for tooth repair and
replacement, In Processing and bonding of dental ceramics,
Woodhead Publishing Limited, Philadelphia, USA, 2013 125-160.
Kaminski HD, Easton AD, Dental Materials Research, Nova
Science, New York, 2009: 1-21.
Yoshinari M, Dérand T. Fracture strength of all-ceramic crowns. Int
J Prosthodont, 7(4), 329-38, Jul-Aug 1994.
Sobrinho LC, Cattel MJ, Glover RH, Knowles JC, Investigation of
the dry and wet fatigue properties of three all-ceramic crown
systems. Int J Posthodont, 11(3), 255-62, May-Jun 1988.
Zhang, Sailer I, Lawn BR, Fatigue of Dental Ceramics, J Dent,
41(12): 1135-47, Dec2013.
Denry I. How and when does fabrication damage adversely affect the
clinical performance of ceramic restorations? Dent Mater, 29(1):8596, Jan 2013.
Atala MH, Gul EB, How to Strengthen Dental Ceramics. Int J Dent
Sci Res, 3(1):24-27, Jan 2015.
Laser Shock Peening and Mechanical Shot Peening Processes
Applicable for the Surface Treatment of Technical Grade Ceramics:
A Review. Pratik P. Shukla1, Philip T. Swanson2and Colin J. Page.
DOI: 10.1177/0954405413507250.
Hondrum SO. A review of the strength properties of dental ceramics.
J Prosthet Dent 1992;67:859-65.
How to Strengthen Dental Ceramics Mustafa Hayati Atala* ,
EsmaBasak Gul. International Journal of Dental Sciences and
Research, 2015, Vol. 3, No. 2, 24-27.
Brukl CE, Ocampo RR. Compressive strengths of a new foil and
porcelain fused to metal crowns. J Prosthet Dent 1987;57: 404-10.
Vrijhoef MMA, Spanauf AJ, Renggi HH. Axial strength of foil, all
ceramic and PFM molar crowns. Dent Mat 1988;4:15-19.
Crispin,
B.J.Contemporary
Esthetic
Dentistry:
Practice
Fundamentals. Tokyo: Quintessence Pub Co.1994.
From Ceramics to Ceramic Steel: Genesis. Udita S Maller, Deepa
Natesan Thangaraj, Sudhakar Maller. 10.5005/jp-journals-100261030.
McLean JW. The development of ceramic oxide reinforced dental
porcelains with an appraisal of their physical and clinical properties.
MDS thesis. University of London 1966.
How to Strengthen Dental Ceramics Mustafa Hayati Atala* ,
EsmaBasak Gul. International Journal of Dental Sciences and
Research, 2015, Vol. 3, No. 2, 24-27.
Anusavice, K.J., Shen, C., Lee, R.B. “Strengthening of Feldspathic
Porcelain by Ion Exchange and Tempering,” J Dent Res, 71 (5),
1134-1138. 1992.
Zaimoğlu, A., Can, G., Fixed Prosthodontics. Ankara: Ankara
University Publishing. 2011. 139-159.
Anusavice KJ, Shen C, Vermost B, Chow B. Strengthening of
porcelain by ion exchange subsequent to thermal tempering. Dent
Mat 1992;8:149.
Asaoka K, Nuwayama N, Tesk JA. Influence of tempering method
on residual stress in dental porcelain. J Dent Res 1992;71:1623.
876
Sanjay Madhavan, et al /J. Pharm. Sci. & Res. Vol. 7(10), 2015, 873-877
[34] Zeng, K., Odén, A., Rowcliffe, D. “Flexure Tests on Dental
Ceramics,”Int J Prosthodont, 9 (5), 434-439. 1996.
[35] Kelly, J.R., Campbell, S.D., Bowen H.K. “Fracture-surface Analysis
of Dental Ceramics,” J Prosthet Dent, 62 (5), 536- 541.1989.
[36] McLaren, E.A. “All-ceramic Alternatives to Conventional
Metalceramic Restorations,” Compend Contin Educ Dent, 19 (3),
307- 308. 1998.
[37]. Wang F, Tooley FV. Influence of reaction products on reaction
between water and soda lime silica glass. J Am Ceram Soc 1958,
41:521-24.
[38] Anusavice KJ, Dehoff PH, Hojjatie B, Gray A. Influence of
tempering and contraction mismatch on crack development in
ceramic surfaces. J Dent Res 1989;68:1182.
[39] Quinn JB, Quinn GD, Kelly JR, et al. Fractographic analyses of three
ceramic whole crown restoration failures. Dent Mat 2005;21:920-29.
[40] Sharmila Hussain, Textbook of dental materials, Jaypee,
Glass/Dental Ceramics, pg.205.
[41] Eroğlu, Z. (2010). “Invitro comparison of fracture strength of
galvano ceramic, metal ceramicandallceramic 3 unit bridge,”
Doctorate Thesis, Erciyes University, Kayseri.
[42] Heuer AH, Lange FF, Swain MV, Evans AG. Transformation
toughening: An overview. J Am Ceram Soc 1986;69:1-4.
[43] Kosmac T, Oblak C, Jernikar P, Funduk N, Marion L. The effect of
surface grinding and sandblasting on flexural strength and reliability
of Y-TZP Zirconia ceramic. Dent Mat 1999, 15:426-33.
[44] Anusavice KJ, Shen, Shawls, Philips science of dental materials 12th
edition Elsevier, Dental Ceramics pg.441.
[45] Wang F, Tooley FV. Influence of reaction products on reaction
between water and soda lime silica glass. J Am Ceram Soc 1958,
41:521-24.
877